Some personal watercraft include therein an engine equipped with a supercharging machine such as a turbo charger or a supercharger. One example is disclosed in Japanese Laid-Open Patent Application Publication No. 2003-26092.
Since the supercharging machine is typically configured to compress air taken in from outside and to supply the compressed air to a throttle device such as a throttle body for controlling an amount of the air supplied to the engine, a pressure in a region of an air-intake passage that is located upstream of a throttle valve in an intake-air flow direction becomes higher than a pressure in a region of the air-intake passage that is located downstream of the throttle valve. If the pressure increases excessively, then a load is likely to be applied to the throttle device, decreasing durability of the throttle device.
In particular, during a partial load operation of the engine, i.e., operation in which the throttle valve is at an intermediate opening degree, the pressure tends to increase because the amount of air required in the engine is less.